Epoxy resin compositions are the preferred polymeric insulating materials for medium and high voltage electrical insulating applications as these provide optimum combination of process and performance properties such as mechanical, thermal and electrical properties. Conventionally cycloaliphatic epoxies cured using acid anhydride hardeners are used for outdoor electrical applications as these enable high resistance to UV weathering. The performance of these compositions is however limited in regions of high humidity involving high precipitation levels and polluted environments where the formation of conductive layer causes considerable reduction in insulation performance leading to increased surface leakage current and arching.
In order to be used in regions having high precipitation levels and polluted environments, epoxy resin compositions should preferably be self-healing and possess hydrophobic properties. Self-healing epoxy resin compositions are able to perform better in such environments as they are able to recover from any damage that may have been caused by environmental factors such as high precipitation and pollution. Hydrophobic properties also help epoxy resins in resisting damage caused under severe climatic conditions.
Since epoxy resins are relatively polar in nature they do not possess hydrophobic properties. Prior art discloses imparting hydrophobic behavior to epoxy resin compositions by addition of pretreated mineral fillers, silicon compounds, polysiloxane/polyether copolymers, hydroxyl terminated polysiloxanes, organopolysiloxane oils. These compounds however demonstrate weak hydrophobic effect and the lack of crosslinking results in phase separation and reduction of adhesion with metals.
In recent years epoxy resin compositions having intrinsic hydrophobicity have been developed by physical or chemical modification of the cycloaliphatic resins with oligomericsiloxanes having terminal glycidyl groups. However, such compositions have inadequate storage stability resulting in shorter shelf life.
Epoxy resin compositions cured using hardeners that have been modified with organopolysiloxanes having epoxy or carbinol functional groups are also known. For example, Japanese Patent Publication Number 2-59185 [59,185/90] and EP 0511670 disclose epoxy resin compositions which contain the reaction product of carboxylic acid-modified organopolysiloxane and epoxy resin. However, such epoxy resin compositions suffer from phase separation and are not stable which limits their commercial use.
Therefore, there is a need for an epoxy resin composition that is self-healing, has intrinsic hydrophobicity and is stable. There is also a need for an epoxy resin hardener that when used for curing an epoxy resin imparts intrinsic hydrophobicity and self-healing property to the resultant epoxy resin composition.